<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>星空</title>
  <style>
    body {
      background: #000;
      overflow: hidden;
    }
  </style>
</head>
<body>
<canvas id="canvas"></canvas>
<script>
  "use strict";

  var canvas = document.getElementById('canvas'),
          ctx = canvas.getContext('2d'),
          w = canvas.width = window.innerWidth,
          h = canvas.height = window.innerHeight,
          hue = 217,
          stars = [],
          count = 0,
          maxStars = 1500;

  // START CANVAS CACHED GRADIENT
  var canvas2 = document.createElement('canvas');
  var w2 = canvas2.width = 100;
  var h2 = canvas2.height = 100;
  var ctx2 = canvas2.getContext("2d");
  // draw a big beefy gradient in the center of the dummy canvas
  var gradientCache = ctx2.createRadialGradient(
          w2 / 2,
          h2 / 2,
          0,
          w2 / 2,
          h2 / 2,
          w2 / 2
  );
  gradientCache.addColorStop(0.025, '#fff');
  gradientCache.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
  gradientCache.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
  gradientCache.addColorStop(1, 'transparent');
  ctx2.fillStyle = gradientCache;
  ctx2.beginPath();
  ctx2.arc(w2 / 2, h2 / 2, w2 / 2, 0, Math.PI * 2);
  ctx2.fill();
  // END CANVAS CACHED GRADIENT

  function random(min, max) {
    if (arguments.length < 2) {
      max = min;
      min = 0;
    }

    if (min > max) {
      var hold = max;
      max = min;
      min = hold;
    }

    return Math.floor(Math.random() * (max - min + 1)) + min;
  }

  function maxOrbit(x,y) {
    var max = Math.max(x,y),
            diameter = Math.round(Math.sqrt(max*max + max*max));
    return diameter/2;
  }

  var Star = function() {
    this.orbitRadius = random(maxOrbit(w,h));
    this.radius = random(60, this.orbitRadius) / 10;
    this.orbitX = w / 2;
    this.orbitY = h / 2;
    this.timePassed = random(0, maxStars);
    this.speed = random(this.orbitRadius) / 30000;
    this.alpha = random(2, 10) / 10;

    count++;
    stars[count] = this;
  }

  Star.prototype.draw = function() {
    var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
            y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
            twinkle = random(10);

    if (twinkle === 1 && this.alpha > 0) {
      this.alpha -= 0.05;
    } else if (twinkle === 2 && this.alpha < 1) {
      this.alpha += 0.05;
    }

    ctx.globalAlpha = this.alpha;
    // draw the cached gradient canvas image
    ctx.drawImage( canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius );
    this.timePassed += this.speed;
  }

  for (var i = 0; i < maxStars; i++) {
    new Star();
  }

  function animation() {
    ctx.globalCompositeOperation = 'source-over';
    ctx.globalAlpha = 0.8;
    ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 1)';
    ctx.fillRect(0, 0, w, h)

    ctx.globalCompositeOperation = 'lighter';
    for (var i = 1, l = stars.length; i < l; i++) {
      stars[i].draw();
    };
    window.requestAnimationFrame(animation);
  }

  animation();
</script>
</body>
</html>
